This invention relates to an electronic time-division-multiplexed private automatic branch exchange (PABX) telephone system, and deals more particularly with such a system which operates essentially without any central or common control and wherein the signalling and supervision functions and the making and breaking of talking path connections are distributed over a number of units, referred to as "appliques", connected to a transmission cable at spaced points therealong and defining stations at which telephones, trunk lines or other pieces of equipment are interfaced with the system.
A PABX is an on-premises telephone system, usually of relatively small size in comparison to telephone company exchanges, intended to service a limited number of stations as found, for example, within a business office or manufacturing plant. Generally, such a system automatically provides for the making and breaking of connections between the various stations of the system and between the stations of the system and one or more trunk lines to the outside world without the need for a switchboard or switchboard operator, although an attendant or operator may often be used to process special calls, such as outgoing and incoming trunk calls.
In the past PABXs have conventionally included central or common control units to which all of the stations of the system are connected by individual lines, the switching equipment for making connections between the various stations and most other major parts of the system, except for the telephones, being part of the common control. As a general practice, present common control units are constructed to service a given maximum number of stations, for example, one hundred stations, and for each potential station, whether used or not, includes a substantial amount of switching equipment exclusively assigned to such station. Therefore, when a common control is used with less than its maximum number of stations, a substantial part of it remains unused and the cost of the system per installed line may become quite high for a system having a number of installed stations considerably less than the maximum number permitted by the common control. Also, the necessity in a conventional PABX system of running individual lines from each subscriber station to the common control makes installation of such a system expensive both in the amount of materials and installation time required, and since the common control is a relatively complex piece of equipment, maintenance is difficult and requires the services of a high skilled technician.
The general object of this invention is to provide a PABX system by means of which the installed cost per line or per subscriber station of the system may be substantially reduced in comparison to coventional PABX systems. This object is achieved by, among other things, eliminating the common control equipment and distributing the switching and signalling and supervision functions of the system throughout a number of appliques each serving an individual station, with the result that only a number of appliques equal to the desired number of stations need be provided with no equipment remaining unused. Also, the appliques are connected to a single transmission cable, containing a relatively low number of conductors, which run from one station to the next so as to reduce installation and/or rearrangement labor and material expense to a minimum.
Another object of the invention is to provide a PABX system which may be selectively adapted, at little expense, to provide a large number of customer service features, some of which are usually not available in conventional PABX systems, such features being, for example, camp-on, fixed forward call transfer, variable forward call transfer, grade of service variation, break-in capability, zone or universal ringing, paging and repertory dialing.
Still another object of the invention is to provide a PABX communication system of the foregoing character made up essentially of semiconductor elements and electronic circuits capable in large part of being reduced to integrated circuit components.
Other objects are to provide a PABX communication system having low initial capital and installation costs, flexibility of growth and features, requiring a minimum of maintenance, of small size, having low power consumption and wherein rearrangement of the telephones can be carried out by the average user without requiring the services of a skilled technician.
Other more detailed objects of the invention will be apparent from the following description of the preferred embodiment of the invention.
In the system illustrated herein a multiconductor cable is described as the transmission medium. However, it should be understood that in its broader aspects the invention is not limited to use with such a cable and instead other types of linear transmission media may be employed. Also, in the illustrated system the timing and communication signals are transmitted on separate lines, but this is not necessary to the broader aspects of the invention, the invention embracing the possibility of transmitting the timing signals and the communication signals on the same line.